The present invention relates to direct electrostatic printing methods in which charged toner particles are transported under control from a particle source in accordance with an image information to form a toner image used in a copier, a printer, a plotter, a facsimile, or the like.
According to a direct electrostatic printing method, such as that disclosed in U.S. Pat. No. 5,036,341, a background electric field is produced between a developer sleeve and a back electrode to enable the transport of charged toner particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field and connected to a control unit which converts an image information into a pattern of electrostatic control fields which selectively open or close the apertures, thereby permitting or restricting the transport of toner particles from the developer sleeve. The modulated stream of toner particles allowed to pass through opened apertures impinges upon an information carrier, such as paper, conveyed between the printhead structure and the back electrode, to form a visible image.
According to such a method, each single aperture is utilized to address a specific dot position of the image in a transverse direction, i.e. perpendicular to paper motion. Thus, the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction.
A new concept of direct electrostatic printing, hereinafter referred to as dot deflection control (DDC), was introduced in U.S. patent application Ser. No. 08/621,074. According to the DDC method each sin gle aperture is used to address several dot positions on an information carrier by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward a paper, and thereby the location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with at least two sets of deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the symmetry of the electrostatic control fields to deflect the modulated stream of toner particles in predetermined deflection directions.
For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having 200 apertures per inch. An improved DDC method, disclosed in U.S. patent application Ser. No. 08/759,481, provides a simultaneous dot size and dot position control. This later method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. According to the method, each aperture is surrounded by two deflection electrodes connected to a respective deflection voltage D1, D2, such that the electrode field generated by the control electrodes remains substantially symmetrical as long as both deflection voltages D1, D2 have the same amplitude. The amplitudes of D1 and D2 are modulated to apply converging forces on toner to obtain smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between D1 and D2. Utilizing this improved method enables 60 xcexcm dots to be obtained utilizing 160 xcexcm apertures.
With or without DDC in direct electrostatic printing methods a plurality of apertures, each surrounded by a control electrode, are preferably arranged in parallell rows extending transversally across the print zone, i.e. at a right angle to the motion of the image receiving medium. As a pixel position on the image receiving medium passes beneath a corresponding aperture, the control electrode associated with this aperture is set on a print potential allowing the transport of toner particles through the aperture to form a toner dot at that pixel position. Accordingly, transverse image lines can be printed by simultaneously activating several apertures of the same aperture row.
However, it can be considered a drawback of current direct electrostatic printing methods that sometimes when printing an image the perceived image density of individual apertures varies over time for the same desired image density. It can also be considered a drawback of current direct electrostatic printing methods that the mechanical precision of interrelating parts of the printer has to be very high.
An object of the present invention is to provide a method of and device for harmonizing the apparent time varying behaviour of different apertures in direct electrostatic printing methods.
A further object of the present invention is to provide a method of direct electrostatic printing which temporally harmonizes the apparent behaviour of individual apertures.
Still a further object of the present invention is to provide a method of and a device for harmonizing a perceived image density with a desired image density in direct electrostatic printing methods.
Yet a further object of the present invention is to provide a method of and a device for decreasing the need for an extremely high mechanical precision during manufacturing of printers working according to direct electrostatic printing methods.
Another object of the present invention is to provide a method of and device for reducing or eliminating perceived uneven image density in direct electrostatic printing methods.
Still another object of the present invention is to provide a method of and a device for trajecting a predetermined, within a predetermined margin, amount of toner/pigment particles to predetermined positions in view of an image which is to be printed.
Yet another object of the present invention is to provide a method of and a device for for reducing or eliminating perceived uneven image density in direct electrostatic printing methods due to mechanical imperfections.
Yet another object of the present invention is to provide a method of and a device for for reducing or eliminating the influence of distance variations between a printhead structure and a pigment source due to mechanical imperfections.
Said objects are achieved according to the invention by providing a direct electrostatic printing device and method for printing an image to an information carrier with increased density harmonization. This is attained by measuring the apparent temporal behaviour of the apertures and subsequently temporally adjusting the control parameters of at least the apertures that seem to temporally diverge during printing. The measurement of the behaviour of the apertures is suitably performed by scanning a known print sample with a predetermined density. The scanned values are inverted around a predetermined value for which no compensation is done to create a two dimensional compensation function. At least the apertures which have an apparent temporal behaviour which diverges from a predetermined behaviour are compensated according to the compensation function of the respective aperture, thereby enabling an increased density harmonization. The compensation function can preferably be signal processed by, for example, a low pass filtering.
Said objects are also achieved according to the invention by providing a direct electrostatic printing device and method for printing an image to an information carrier with increased density harmonization during printing. This is attained by measuring undesired image density variations in a direction parallel to the relative movement between an image receiving member and a printhead structure. The density variations are caused by distance variations between the printhead structure and a pigment particle source during printing, which at least in part is caused by a relative movement between at least a part of the pigment particle source and the printhead structure. A control unit is arranged to control the transport of pigment particles in such a way as to compensate for these undesired image density variations during printing, thus attaining a percepted uniform printed image density along a printed image for a specific desired image density.
Said objects are also achieved according to the invention by providing a direct electrostatic printing device and method for printing an image to an information carrier with improved alignment between printed part images from two or more print stations. The improved alignment is based on a basic mechanical alignment with a basic accuracy between the print stations which is improved by an electronic alignment of the corresponding bitmaps. The electronic alignment is made possible by the capability of at least one print station to print at least one additional dot in relation to the corresponding bitmap. The minimum number of additional dots being dependent on the attained basic accuracy of the basic mechanical alignment.
Said objects are also achieved according to the invention by providing a direct electrostatic printing device according to claim 1. The dependent claims 2 to 23 disclose advantageous embodiments of the invention.
Said objects are also achieved according to the invention by a method for printing an image to an information carrier according to the steps of claim 24. Further method variations of the method according to the invention are possible according to previously described enhancements in view of the application of the invention according to claims 2 to 23.
The present invention satisfies a need for density harmonization not previously met.
The present invention relates to an image recording apparatus including an image receiving member conveyed past one or more, so called, print stations to intercept a modulated stream of toner particles from each print station. A print station includes a particle delivery unit, a particle source, such as a developer sleeve, and a printhead structure arranged between the particle source and the image receiving member. The printhead structure includes means for modulating the stream of toner particles from the particle source and means for controlling the trajectory of the modulated stream of toner particles toward the image receiving member.
According to a preferred embodiment of the present invention, the image recording apparatus comprises four print stations, each corresponding to a pigment colour, e.g. yellow, magenta, cyan, black (Y, M, C, K), disposed adjacent to an image receiving member formed of a seamless transfer belt made of a substantially uniformly thick, flexible material having high thermal resistance, high mechanical strength and stable electrical properties under a wide temperature range. The toner image is formed on the transfer belt and thereafter brought into contact with an information carrier, e.g. paper, in a fuser unit, where the toner image is simultaneously transferred to and made permanent on the information carrier upon heat and pressure. After image transfer, the transfer belt is brought in contact with a cleaning unit removing untransferred toner particles.